Classifying solids by conductivity

Solids can be sorted by conductivity σ, or by its reciprocal, resistivity ρ = 1/σ.

Metals: ρ ~ 10⁻² to 10⁻⁸ Ω m, so σ ~ 10² to 10⁸ S m⁻¹.

Semiconductors: ρ ~ 10⁻⁵ to 10⁶ Ω m, so σ ~ 10⁵ to 10⁻⁶ S m⁻¹, in between metals and insulators.

Insulators: ρ ~ 10¹¹ to 10¹⁹ Ω m, so σ ~ 10⁻¹¹ to 10⁻¹⁹ S m⁻¹.

These ranges only indicate orders of magnitude; real materials can fall outside them. Resistivity alone does not settle the class, and the band picture gives the deeper difference.

Elemental semiconductors: silicon (Si) and germanium (Ge).

Compound semiconductors can be inorganic (CdS, GaAs, CdSe, InP), organic (anthracene, doped phthalocyanines) or organic polymers (polypyrrole, polyaniline, polythiophene).

Most devices today use Si, Ge or inorganic compounds. Since 1990 a few devices built on organic and polymer semiconductors have appeared, the beginnings of polymer and molecular electronics.

The chapter sticks to inorganic, mainly elemental, semiconductors; the same ideas largely carry over to compound ones.